This project has three principal objectives: (1) to specify the membrane transport and cellular mechanisms that help control the rate, direction and composition of the fluid transported by the pigment epithelium (PE); (2) to determine the PE transport mechanisms that help control calcium homeostasis in the subretinal and choroidal spaces; (3) to measure the O2 consumption of this metabolically active tissue with and without the retina attached, in the light and dark. An important hypothesis to be tested is that control of fluid secretion is mediated by cAMP activated increase in cell membrane C1 conductance. This idea will be examined in a series of microelectrode experiments. The fluid transport measurements will be carried out using the isolated PE-choroid of the frog and cat. Other experiments will determine which Ca transport mechanisms (e.g., ATP dependent pumps, exchangers, passive channels) control the movements of Ca across the apical and basal membranes of the PE. A series of transport and electrophysiological experiments will examine the interactions between the active and passive components of the Ca, K, and taurine transport systems. Finally, the O2 consumption of the PE-choroid will be measured and the dependence on active transport will be determined. The relationship between retinal activity and choroidal O2 tension will be studied using the isolated retinal PE-choroid preparation (frog). An O2 microelectrode will be used to monitor retinal activity in the light and dark, and the response of this electrode will be measured as a function of choroidal O2 tension. The life of the retina depends on a healthy, normal relationship to the pigment epithelium. In physiological terms, this means that O2, water, ions and metabolites are transported by the epithelium between the retinal and choroidal spaces and that changes in retinal activity and blood flow can modulate the activity of the epithelium. The proposed experiments are specifically designed to quantify this relationship when it is normal and when it has been severely altered. Hence, this work could have considerable significance for understanding diseases which affect the epithelium or the choriocapillaris.